Many products, when subjected to freezing conditions, deteriorate rapidly to the point where they are seriously affected by loss of quality, loss of activity or are rendered totally unusable. A vivid example of such deterioration is soluble vaccines. In a 1996 report published in the Bulletin of the World Health Organization (74, 391-397) it was disclosed that 99% of a shipment of Hepatitis B vaccines was rendered useless because of a freeze/thaw cycle that occurred somewhere in the supply chain. Such de-activation of the vaccine usually goes undetected thereby resulting in zero protection for injected adults and children. Other examples where freezing has a detrimental effect include: medicines, human organs, pharmaceuticals, foods such as mayonnaise, fabric softeners, latex products such as paints, concrete modifiers, laboratory supplies, flowers etc. Biological samples such as whole blood, insulin and the like can be seriously affected or lost when subjected to freezing conditions (or near freezing conditions), and this is also true for high value biotechnology drugs containing proteins, enzymes or peptides. Therefore, it is desirable to the seller, buyer and end user of such products that some indicator means be provided which will signal a change in products caused by freezing conditions.
It is not only freezing conditions that potentially degrade certain products. In the case of vaccines, these are optimally stored at 2-8° C. Therefore, traditional freeze indicators, which provide an indication that temperatures have fallen below the freezing point of water, are not, in fact, optimal for this purpose.
A lower threshold indicator is potentially useful in preparing and serving hot food, especially in a professional setting. Once hot food has been prepared for serving, it is desirable to know if the temperature of this food has fallen below a lower threshold temperature at which bacteria propagate. For example, it may be desirable to indicate whether the hot food has fallen to a temperature below about 60° C., and, more desirably, to indicate a period over which the food has been left to stand below this lower threshold temperature.
A number of freeze indicators are known in the art. Problems associated with these prior art devices include high cost including reliance on expensive materials such as gold, slow response time, size, limited shelf life and lack of clarity in indication.    PTL 0001: U.S. Pat. No. 4,846,095 A (EMSLANDER). 1989 Jul. 11.
describes a device for indicating that an ambient temperature has fallen to or below a predetermined lower threshold temperature, comprising a porous membrane in contact with a mixture of two liquids held within a receptacle. On partial freezing of one component the remainder can wet the membrane.    PTL 0002: U.S. Pat. No. 4,457,253 A (MANSKE). 1984 Jul. 3.
describes a device that uses fatty acid esters for their versatile freezing points. The fatty acid is a liquid at the end of the capillary tube and it shrinks in order to draw a dyed compound into a plug. There is a separating liquid (immiscible with the dye and the fatty acid) to stop the plug being prematurely soaked.
It would be desirable to provide a lower threshold indicator device, which overcomes or at least ameliorates some of the shortcomings of the prior art devices. In particular, it would be desirable to provide a lower threshold indicator device which is inexpensive, provides excellent binary indication to end users and responds rapidly to temperatures below the lower threshold. It would be further desirable to provide a lower threshold indicator device which can be activated for use by the end user. It would be further desirable to provide a lower threshold indicator device which provides an indication of a period of time spent at or below the lower threshold temperature.